Frame synchronization circuit

ABSTRACT

Disclosed is a mobile communication system including a network constituted of at least one switching center and a plurality of base stations, and a mobile station which communicates with the base stations simultaneously. The system permits varying transmission delay between the switching center and the base stations according to the type of services available to the mobile station. The object of the present invention is to propose a communication which permits varying transmission delay according to the type of service currently employed, and to promptly recover a synchronization state even if an out-of-sync state happens. To attain the object, a memory means (mobile switching center processor  32 ) stores transmission delay characteristics corresponding to services which are available to the mobile station. Furthermore, a communication timing setting means (diversity handover trunk  34 ) determines the timing of communication for the base stations according to the transmission delay characteristic selected according to the service.

This application is a continuation of U.S. application Ser. No.11/223,450 filed Sep. 8, 2005, which is a continuation of U.S.application Ser. No. 09/125,958 filed Aug. 26, 1998, now U.S. Pat. No.6,977,903, which is a nationalized PCT application, PCT/JP97/04834 filedDec. 25, 1997, which claims priority to Japanese application No.H08-348900 filed Dec. 26, 1996, the content of all of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a frame communication system which issuitably used for altering transmission delays according to the servicetypes in Type 5 and Type 2 transmission of packet mode, frame relay modeand ATM (Asynchronous Transfer Mode).

TECHNICAL BACKGROUND

A diversity handover communication technique, wherein a mobile stationmoves across the boundary between cell sites of different base stationswhile continuing to communicate with the base stations, is described inJapanese patent application No. 6-106953. This application states amethod in which a base station creates reliability information based onthe state of radio frames received from a mobile station, and attachesthe reliability information to each radio frames. Then, a diversityselection process is carried out in the communication network.

Japanese patent application No. 6-210193 discloses another diversityhandover method in which when communication is made between a mobilestation and a superior system, frame identification information is used,to prevent the occurrence of skipping or overlapping of frames duringdiversity selection of the frames due to the difference in delay offrame transmission through different base stations, and thus securediversity handover is ensured.

However, these methods have following problems.

(1) In the method disclosed in Japanese Patent Application No. 6-210193,when a mobile station (MS) makes a communication through a mobileswitching center (MSC), frame identification numbers are used to absorbdifferences in delay arising during the passage of frames throughdifferent base stations, and maximal-ratio combining or diversityselection of resulting frames is achieved. For MS to absorb differencesin delay of downlink frames, it is necessary for MS to have a bufferwith a considerably large capacity. This makes it difficult to reducethe size of a responsible terminal. Further, as this method requiresframe identification information to be exchanged between different radiozones, the communication system it promises to realize will beinefficient because it will be not able to effectively exploit thecapacity allowed to radio routes.

(2) In the conventional frame receiving systems, there was no attentionpaid for the difference in delay of frame transmission according to thetype of involved service, and thus sets a fixed maximal transmissiondelay independent of the type of service currently involved.Accordingly, even when a transmission mode is introduced which allowsdifferent transmission delays according to the type of service (forexample, Type 5 or Type 2 of ATM), a receiver must respond with a fixed,too long delay to frames of service which does not require such a longdelay.

(3) The conventional frame receiving system regards as fixed the maximaltransmission delay arising as a result of the passage of frames throughnodes and links, and thus it can not meet the situation where anunexpected transmission delay arises owing possibly to changes intransmission state or in traffic. It causes disconnection of thecommunication in the presence of such delay.

(4) In the conventional handover process, as communication quality issolely determined by the transmission condition through the radio link,it can be monitored by the radio receiver connected to the link.However, in diversity handover, communication quality is obtained as theoutcome of maximal-ratio diversion or diversity selection of frames fromall branches involved in the handover, and thus it can not be monitoredonly by a radio receiver.

Maximum ratio combining of frames is a technique whereby MS receivesdownlink frames from a plurality of BSs, and combines received signalsin such a way as to improve communication quality by site diversityeffect. This technique is also utilized by a single BS which combinesuplink frames from MSs incoming through a plurality of TRXS.

Namely, in handover involving a plurality of sectors in a zone governedby a BS (intracellular, inter-sector diversity handover), combining ofuplink radio frames is performed by the BS according to maximal-ratiocombining.

On the other hand, diversity selection is applied to combining of uplinkradio frames in diversity handover involving a plurality of BSs. Uplinkradio frames coming by way of a plurality of BSs are given reliabilitydata different according to the routes they pass, and a diversityhandover trunk chooses a frame having the best reliability information.

The reason why maximal-ratio combining is not applied for the combiningof uplink radio frames in handover involving a plurality of BSs is toprevent transmission of a vast amount of information required formaximal-ratio combining through routes connecting the plurality of BSsand MSC, and thereby to prevent congestion of traffic. Diversityselection, as compared with maximal-ratio combining, does not requiremuch reliability information for combining, although it allows only alow gain.

(5) With the conventional technique, when an out-of-sync(out-of-synchronization) state arises, BSs, whenever they detect it,inform of it to MSC processor through their own control linksindependently of each other. In diversity handover system, control ismade such that the power required for transmission of uplink frames fromMS becomes most efficient for a certain BS. Therefore, the other BSswhich are not objects of power control may often inform MSC ofout-of-sync states. Thus, a vast amount of control signals aretransmitted through routes connecting BSs and MSC processor, and anoverload is imposed on the processor.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide a framecommunication system that permits varying transmission delay accordingto service types.

Therefore, in one aspect of the present invention, a frame transmittingdevice comprises: a frame-synchronize-information adder for addingframe-synchronize-information to a user frame; and a transmitter fortransmitting the user frame with the frame-synchronize-information.

In another aspect of the present invention, a frame receiving devicecomprises: a receiver for receiving the user frame with theframe-synchronize-information; and a frame synchronizer for executingframe synchronization referring to the frame-synchronize-information.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a communication system according to anembodiment of the present invention.

FIG. 2 is a block diagram showing important structural elements of amobile switching center 3 of the system in FIG. 1.

FIG. 3 is a block diagram showing important structural elements of abase station 2 of the system in FIG. 1.

FIG. 4 is a connection management table.

FIG. 5 is an MSC-BS delay time management table.

FIG. 6 is a diagram showing quality degradation measurement parametersand out-of-sync detection parameters.

FIG. 7 is a table representing traffic information managed by a MSCprocessor 32.

FIG. 8 is a diagram showing the operation necessary for the qualitymeasurement by means of an up-down counter.

FIGS. 9 and 10 cooperate to form a flowchart representing the qualitymeasurement operation using with the up-down counter.

FIGS. 11 and 12 cooperate to form a sequence diagram representing adiversity handover procedure.

FIGS. 13 and 14 cooperate to form a sequence diagram representing abranch switching handover procedure.

FIGS. 15 and 16 cooperate to form a sequence diagram representing anannouncement and management procedure for announcing quality degradationand out-of-sync state at beginning and ending of communication.

FIGS. 17 and 18 cooperate to form a diagram illustrating configurationsof frames passing between individual nodes.

FIG. 19 is a diagram illustrating the operation necessary for thediversity selection treatment of user frame.

FIG. 20 is a diagram illustrating the operation of inter-MSC diversityhandover.

FIG. 21 is a flowchart representing an uplink transmission procedure;

FIG. 22 shows a classification of handover techniques in terms ofcontrol range;

FIG. 23 is a diagram showing the states of handover branches classifiedby handover branch control.

(Notes)

*1: Simultaneous control (addition, deletion or addition/deletion) ofplural Brs is possible in response to a single request for DHO triggerfrom MS.

*2: When MS determines the maximal connectable Brs to be 3, “deletionmay turn into addition”.

FIG. 24 is a table showing, as an example, the correspondence of thehandover trigger activated during mobile communication with the type ofhandover.

FIG. 25 is a table showing, as another example, the correspondence ofthe handover trigger activated during mobile communication with the typeof handover.

FIG. 26 is a diagram used for describing operations for calculating aradio frame offset number OFS and radio frame number FN.

FIGS. 27 and 28 form a timing chart showing procedures in variousdevices.

FIGS. 29 and 30 are tables representing examples of calculationprocedures of timing-parameters.

FIG. 31 is a diagram illustrating the operation of branch switchinghandover.

FIG. 32 is a diagram showing, as an example, a management table ofparameters necessary for FN slide processing.

FIGS. 33 and 34 are diagrams illustrating the operation of uplink FNslide processing.

FIGS. 35 and 36 are diagrams illustrating the operation of uplink FNslide processing.

FIG. 37 is a diagram used for describing the operation of a modificationof the embodiment;

FIG. 38 is a diagram illustrating the inter-MSC handover.

FIG. 39 is a block diagram showing the constitution of MSCs.

BEST MODE FOR CARRYING OUT INVENTION

1. Structure of an Embodiment

Below the structure of an embodiment of this invention will be describedwith reference to FIG. 1.

In FIG. 1, symbols 1 and 10 represent mobile stations (MS); symbols 2and 4-9 base stations; and symbols 3 and 11 mobile switching station(MSC), and they form nodes in a mobile communication system.

Within the base station 2, symbol 23 represents an MSC interface (MIF)installed in the BS, and form a communication link and a signal linkwith a BS interface (BIF) 33 installed in MSC 3. A radio framesynchronizer (MFC-B) 21 installed in the BS determines framesynchronization in BS 2, and provides an operation reference clock toevery section in BS 2.

A transceiver (TRX) 25 transmits and receives radio frames to and fromthe mobile station 1. A modulator/demodulator. (MDE) 24 modulates anddemodulates the radio frames and corrects errors thereof. A base stationprocessor (PRC) 22 controls elements of the base station 2 on the basisof a predetermined control program. Each of the other base stations 4-9has the same structure as that of the base station 2.

Next, in the mobile switching center 3, a switching unit (SW) 38 isprovided for switching transfer routes of frames in the mobile switchingcenter 3. A frame synchronizer (MFC-M) 31 manages to synchronize frameoperations in the mobile switching center 3 and provides reference clockpulses to elements in the mobile switching center 3 as similar to theframe synchronizer 21 of the base station 2. An MSC processor (PRC-M) 32controls elements in the mobile switching center 3 as similar to theprocessor 22 of the base station 2.

In the embodying system, communication between the mobile stations 1 and10 and the base stations 2 and 4-9 is carried out according to CDMAtechnique. In accordance with CDMA, it is possible for the mobilestations 1 and 10 to communicate with a plurality of base stations usingwith the same frequency band for a radio channel. Therefore it ispossible to conduct diversity maximal-ratio combining process anddiversity selection process in order to improve communication qualityand to minimize the congestion in the radio channel.

This is a communication technique, with respect to downlink radioframes, an MS receives radio waves from a plurality of BSssimultaneously and applies the maximal-ratio combining to them, whilewith respect to uplink radio frames, a diversity handover trunk choosesthe radio frames of BS which is in a better communication state with theMS.

Symbol 34 designates a diversity handover trunk (DHT) which executesframe sync adjustment and controls handovers across a plurality of BSs.DHT 34 absorbs fluctuations in uplink radio frames through a pluralityof routes, and then makes a diversity selection.

Namely, DHT 34 waits frames up to a certain delay time set within thesystem, to transmit them, and the delay time is so set as to absorbdelays in transmission of frames through individual routes.

Symbol 35 designates a high efficiency speech coder (VXC) which executestranscoding or others to speech user frames. A data service controlsystem (DSC) 36 executes transcoding or others to data service frames. Arelay network interface system 37 communicates various signals with acommunication relay network, signal relay network, sync relay network,or the like not illustrated here.

Control signals provided by BS processor 22 of BS 2 to MSC processor 32of MSC 3 are transmitted by way of BS processor 22, MSC interface 23 inBS, and BS interface 33 in MSC.

Control signals provided by MSC processor 32 to BS processor 22 aretransmitted in the reverse order as above. Control signals provided byMS 1 to BS processor 22 is transmitted by way of BS 1, radio transceiver25, BS modulator/demodulator 24 in order. Control signals provided by BSprocessor 22 to MS 1 is transmitted in the reverse order as above.

In addition, control signals provided by MS 1 to MSC processor 32 of MSC3 is transmitted to MSC processor 32 by way of a radio transceiver 25,BS modulator/demodulator 24, interface 23 of BS, interface 33 of MSC,and diversity handover trunk 34. Furthermore, information provided byMSC processor 32 to MS 1 is transmitted in the reverse order as above.

2. Operation of the Embodiment

2.1. Synchronization Setting of Radio Frames

In each of the nodes (BSs 2 and 4-9, and MSCs 3 and 11) of the mobilecommunication network illustrated in FIG. 1, frame synchronizer 21 or 31carries out synchronization adjustment for the frames in thecorresponding node.

In the synchronization adjustment in the nodes, the allowable phasedifference of radio frames is determined to be less than a half of aninterval of the radio frames, which are transmitted between the mobilestation 1 and the base station 2, in order to prevent a largetransmission delay of radio frames. For example, if the radio frameinterval is 10 msec, the allowable phase difference is less than 5 msec.As long as the phase difference is shorter than the allowable limit, allthe involved nodes (BSs 2 and 4-9, and MSCs 3 and 11) can synchronize.

Frame synchronizers 21 and 31 deliver reference clock pulses to everyelement in their respective nodes. In the embodiment, the cycle of thereference clock pulses is 0.625 msec. A period equal to 16 times thereference clock cycle is called a radio frame clock unit (morespecifically, it is equal to 0.625×16=10 msec).

In addition, a number is determined by each radio frame clock unit. Thenumber, called frame number FN, is incremented from 0 to 63 in a cyclicmanner. In a single radio frame clock unit, a number is determined byeach clock pulse. The number, called radio frame offset number OFS, isincremented from 0 to 15 in a cyclic manner.

In FIG. 1, radio frame synchronization adjustment between differentnodes is achieved using with wired communicating routes, since it ispossible that the base stations may be situated where they can notreceive radio wave. However, the radio frame synchronization adjustmentmay be achieved using with a certain wireless means such as GPS.

The “synchronization” and “phase difference”, which are described inthis specification, will be understood by the following explanation incomparison with commonly used clocks.

All clocks in the world tell passage of 24 hours in a day, and have thesame cycle. However, when two clocks at two spots between which there isa time difference are compared, the indications of the clocks aredifferent from each other. The time difference can be regarded as the“phase difference”.

This difference is basically maintained at any time although some errorsoccur due to the precision of the clocks. Accordingly, it can be saidthat the two clocks “synchronize” with each other with a certaindifference maintained.

2.2 Onset of Communication

2.2.1. Call Dispatch and Link Setting

When a call is dispatched from MS 1, or a call is dispatched from astation outside or within the network (not illustrated here) to MS 1,control signals are exchanged between MS 1, BS processor 22 and MSCprocessor 32, and communication resources which may be requiredaccording to the type of service are hunted and activated.

At the same time, communication links and associated control links toconnect communication resources are established within the mobilecommunication system. Here the communication link, when used for speechcommunication, is a link connecting MS 1, radio transceiver 25, BSmodulator/demodulator 24, interface 23 of BS, MSC interface system 33,diversity handover trunk 34, high efficiency speech coder 35 and relayinterface system 37 in order.

On the other hand, the communication link, when used for datacommunication, is a link connecting the same elements as above excepthigh efficiency speech coder 35 being replaced with a data servicecontrol system 36. The associated control link is a link connecting MS1, radio transceiver 25, BS modulator/demodulator 24, interface 23 ofBS, BS interface 33 of MSC, diversity handover trunk 34 and BS processor32.

This associated control link which is installed to be attached to thecommunication link is utilized for setting the second call during theonset or progression of communication, setting radio routes between a MSand BS, and controlling handovers, radio transmission, and mobility.

Referring to FIGS. 17 and 18, transmission frames of individual segmentswill be explained with attention being paid to their names andconfigurations. In this example, communication through wired routesbetween a BS and MSC takes place on the basis of AAL Type 2 of ATM (asspecified in the ITU-T I. 363.2 draft recommendation), but the modeproposed by this embodiment can be applied with the same profit tocommunications in packets and frame relays, and on other AAT Types ofATM.

Explanation will be given taking, as an example, how uplink processingis achieved by individual systems. A user frame, after being dividedinto 10 msec units, undergoes encoding and modulation in an MS to betransmitted as a radio frame. The radio frame is received by a BS and,after being demodulated and decoded, is given radio frame numbers andreliability information. The detail of radio frame number FN andreliability information is shown in FIG. 19.

The transmission frame communicated between BS and MSC is called aBS-MSC frame. When communication between a BS and MSC takes place on thebasis of Type 2 of ATM, radio frames comprising speech with a small userframe length (45 octet or less) and transmitted through a low speedradio route can be accommodated by one Type 2 CPS packet, while radioframes comprising data with a large user frame length (over 45 octet)and transmitted through a high speed radio route can not be accommodatedby one Type 2 CPS packet, and divided into a plurality of BS-MSC framesfor transmission. In one example, a radio frame is divided into threeportions, each of which is transmitted as Type 2 CPS packets.

A diversity handover trunk receives wired frames, execute diversityselection of the frames per each BS-MSC frame, and sends the results, asthe intra-MSC frames, to a service trunk such as the high efficiencyspeech coder 35 and data service control system 36. Intra-MSC frames arereconverted by the service trunk into user frames, processed asappropriate according to intended services, and transmitted as relayframes in a form adaptive to a subsequent relay network.

2.2.2 Parameter Setting

Then, referring to FIGS. 2 and 15, the operation of diversity handovertrunk 34 will be described in detail.

First, a communication controller 32-1 in the MSC processor 32 informs aDHT controller 34-1 in a hunted (inserted into the link) diversityhandover trunk 34, of quality degradation parameters, out-of-syncdetection parameters, timing correction parameters, DHO branchinformation, network-side connection identifiers, and trafficinformation.

Examples of quality degradation measurement parameters and out-of-syncdetection parameters are shown in FIG. 6. Exemplified contents oftraffic information are shown in FIG. 7. The quality degradationmeasurement parameters include the cycle of measurement of qualitydegradation and threshold value which should be announced as occurrenceof degradation. Furthermore, the out-of-sync detection parameter is anumber of successive non-synchronized cells. If the number is countedup, the out-of-sync state is recognized.

The traffic information carries the intervals of arrived cells and thenumber of received cells at a given timing when ATM is applied forcommunication through a wired route between a BS and a MSC. Theseparameters and data are managed by MSC processor 32 according toindividual services.

Furthermore, the timing correction parameter includes a correction valuefor uplink/downlink frame number, and a correction value foruplink/downlink frame offset number. These numbers are calculated basedon an MSC-BS delay time management table shown in FIG. 5 stored in amemory 32-2. Each of the delay time values in FIG. 5 includes 5 msec,which is the maximum phase difference allowed for transmission betweenthe MSC and the corresponding BS. Furthermore, if another MSC isinserted between each base station and mobile switching center 3, adelay caused by the insertion of the inserted mobile switching centershould be included in each delay time value in FIG. 5.

Next, referring to FIG. 26, explanation will be given of the method howto compute the correction values for uplink/downlink radio framenumbers, and correction values of uplink/downlink radio frame offsets.Considering firstly downlink frames;

-   (1) DHT in an MSC allocates frame number FN after having added a    maximal fluctuation delay to a reference clock timing created by    MFC-M, and transmits the frames to a BS. The thus transmitted frames    are received by BS; and-   (2) an MDE of BS converts the frames referring to frame number FN    and in accordance with a reference clock timing created by MFC-B and    offset timing, adjust them, and sends them to radio communication    zones as a series of radio frame numbers.

On the other hand, considering the uplink radio frames;

-   (3) the radio frames are received by TRX of a BS in accordance with    a reference clock created by MFC-B, and are given by MDE radio frame    numbers FNs created by MDC-B, and transmitted to an MSC; and-   (4) The thus transmitted frames are received by DHT in the MSC which    allocates frame number FNs after having added a maximal fluctuation    delay to a reference clock timing created by MFC-M, and transmits    the resulting frames to a subsequent system.

Next, an exemplified method for calculating the above parameters will beexplained, assuming that the mobile station 1 executes diversityhandover when the base stations 2 and 4 transmit voice frames to themobile station 1. The MSC-BS delay time management table shown in FIG. 5indicates that the BSs 1 and 2 (base stations 2 and 4) allow the delaytime of 30 msec and 38 msec respectively for this case. Therefore, 38msec should be selected as the maximum transmission delay.

Namely, to nullify the fluctuation of radio frames arriving from thebase stations 2 and 4, the maximum transmission delay at an uplink frameextraction controller 34-8 is set at 38 msec. However, if execution ofdiversity handover is not limited to all the base stations, and if thefluctuation of radio frames should be nullified for all the basestations, the maximum transmission delay should be set at 40 msec thatis the maximum value in the table.

38 msec approximately equal to three radio frame clock units (30 msec)and 13 radio frame offset units (8.125 msec). Accordingly, thecorrection number for uplink frame number and the correction number foruplink frame offset number are set at “3” and “13”, respectively. Thecorrection numbers for downlink frame number and downlink frame offsetnumber are also set at “3” and “13”, respectively.

However, if uplink and downlink lines have different delaycharacteristics, different values for the uplink and downlink lines maybe stored in the MSC-BS delay time management table of FIG. 5. In thiscase, for the uplink and downlink lines, different correction numbersfor radio frame numbers and frame offset numbers may be set according tothis table.

Correction is achieved, with respect to the reference clock deliveredfrom the synchronizer 31 of MSC, by subtracting the uplink radio framenumber correction value and radio frame offset correction value from theclock. On the other hand, for the downlink radio frame number correctionvalue and radio frame offset correction value, correction is achieved byadding those correction values to the reference clock.

The DHO branch information includes the number of lines connected to thediversity handover trunk 34, and connection identifiers. Thenetwork-side connection identifier refers to the connection identifieron the network side which is connected to the diversity handover trunk34. These data are described in a connection management table shown inFIG. 4 and managed by the MSC processor 32, and are used for determiningthe number of connections and identifying frames when uplink frames areselected or when downlink frames are distributed to the base stations.

2.3. Processing to Downlink Frames in MSC 3

When downlink intra-MSC frames appropriately divided to conform to radioframe length are provided from the network 12 through the interface 37,the intra-MSC frames are received by a downlink frame receiver 34-2.

Then, in a downlink frame extraction controller 34-3, extraction of theintra-MSC frames thus received is executed. The timing for extractioncorresponds with the timing corrected on the basis of the downlink radioframe offset correction value which is sent by DHT controller 34-1.

Namely, the intra-MSC frames are extracted according to the timing whichis determined after the downlink frame offset correction value has beensubtracted from “16”. For example, if the downlink frame offsetcorrection value is “13”, the intra-MSC frame corresponding to the thirdreference clock pulse in one radio frame clock unit is extracted since16−13=3.

Furthermore, the number of cells and the interval of cells to beextracted as intra-MSC frames are determined according to trafficinformation. The cell interval is basically n times the interval ofradio frames wherein n is an integer. When intra-MSC frames areextracted by a downlink frame extraction controller 34-3, a downlink FNadder 34-4 adds radio frame numbers FN to the intra-MSC frames.

The radio frame number FN is obtained in the following manner. Thecorrected downlink frame number, “3” in the above example, and thecorrected radio frame offset number, “1”, are added to the radio framenumber FN determined by the reference clock pulses provided by the MSCframe synchronizer 31. Then, the result is divided by “64” and theresidue is the radio frame number FN.

Consequently, in this embodiment, the downlink frame receiver 34-2executes a timing correction of the reference clock pulses on the basisof the corrected downlink frame offset number, while the downlink FNadder 34-4 executes a correction of radio frame clock units.

Then, the BS extracts the downlink frames according to the radio framenumbers FN determined by the reference clock pulses provided by BS radiosynchronizer 21 with the radio frame offset correction value being setat “0”. Therefore, the extraction of downlink frames in the BS is easilyachieved.

Next, a downlink frame copier 34-5 make copies of intra-MSC frames basedon DHO branch information, depicted in FIG. 4, provided by a DHTcontroller 34-1, so that the number of copies is equal to the number ofbranches involved in diversity handover. To the copies of the frames,which are BS-MSC frames, attached are connection identifierscorresponding to the branches, the connection identifiers being used foraddress information of user frames.

In the example depicted in FIG. 1, diversity handover is executed to thetransmissions to the MS 1 through the BSs 2 and 4, so that the branchnumber is “2”. Furthermore, if the intra-MSC frames and wired frames arecontained in ATM cells to be transmitted, then all cells are copiedonce, and the connection identifier identifying the BS 2 is attached toeither series of the original cells or series of copies while theconnection identifier identifying the BS 4 is attached to the otherseries of the original cells or series of copies.

BS-MSC frames thus copied as appropriate are sent to a downlink framedeliverer 34-6. Then, BS-MSC frames are delivered based on theconnection identifiers by way of interface 33 of MSC to individual wiredbranches, that is, to BSs 2 and 4.

2.4. Processing to Downlink Frames in BS

Next, with reference to FIG. 27, processing after the arrival of theBS-MSC frames to the BS 2 from the to-BS interface 33 of the MSC 3 willbe described. The downlink BS-MSC frames are received by the to-MSCinterface 23 of the BS 2, and then transferred through a downlink framereceiver 24-1 to a downlink frame extraction controller 24-2. In thedownlink frame extraction controller 24-2, a downlink BS-MSC frame isextracted from the received BS-MSC frames according to the referenceclock pulses provided by the BS radio frame synchronizer 21.

Extraction of BS-MSC frames at BS (BS 2 in above example) which acts asa reference for communication synchronization during the onset ofcommunication takes place with the radio frame offset value OFS ofreference clock being set to “0.” If there are no BS-MSC frames that canbe extracted according to above timing, waiting time is prolonged to thenext timing (after “1” radio frame clock cycle) and extraction of BS-MSCframes is resumed.

On the other hand, in the subordinate BS 4 accommodating a branch whichis added for diversity handover at the onset of communication or duringcommunication, a processing is executed to put the timing of radiosignal communication there with the timing of radio frames transmittedor received by master BS which acts as a reference for synchronizationwhen communication with MSs is performed.

When involved communication nodes constituting a mobile communicationnetwork adjust synchronization phases using wired routes so that phasedifference in synchronization would be less than 5 msec, for a given MSto execute a maximal-ratio combining processing, it is necessary to havea buffer with a sufficiently large capacity to nullify synchronizationvariations up to 5 msec, because radio frames coming from other MSsengaged in diversity handover have synchronization variations up to 5msec.

However, enlargement of buffer size would conflict with contracted MSsize, and thus it is necessary for the subordinate MS to adjust theradio frame offset value around “0” so that the sync errors, whichotherwise would be 5 msec at maximum, may become about “0.625 msec” atmaximum.

The radio frame synchronization phase difference between the master BSwhich acts as a reference for communication synchronization and thesubordinate BS is determined when MS starts diversity handover. Namely,radio frames which are currently handled by MS, and radio frames from anannouncement channel of the subordinate BS which are newly handled arecompared so that the phase difference between the two may be checked.

The checked result is transferred, by way of MSC, to the subordinate BS.It is possible to finely adjust the radio frame offset value of thesubordinate BS based on this measurement. When this fine adjustmentexceeds the length of one radio frame clock unit, the radio frame numberFNs of the same BS are shifted in association.

Turn back to FIG. 3. BS-MSC frames thus extracted are provided to adown-frame processor 24-3, where encoding treatment for prevention ofthe entry of errors during transmission over radio link and modulationfor radio transmission are executed, to establish radio frames. Then,thus established radio frames are transmitted, by way of the transceiver25, to the zones of involved BSs.

When MS 1 is engaged in diversity handover, it receives radio framesfrom BSs 2 and 4. Then, it applies maximal-ratio combining to them, andtailors them into user frames.

The downlink frame receiver 24-1 monitors radio frame number FNs givento BS-MSC frames and stored in its buffer, and announces the appearanceof “frame delay” when it detects a sufficiently long delay in thearrival of BS-MSC frames carrying radio frame number FNs to beextracted, in association with the downlink frame extraction controller24-2. When such announcement is received, the BS delivers “a request forFN correction” to diversity handover trunk 34.

When the downlink FN correction request is provided to diversityhandover, trunk 34, the DHT controller 34-1 renews the downlink framenumber correction value. The renewed downlink frame number correctionvalue is transferred to a down-frame FN adder 34-4, and allocation ofradio frame number FNs to subsequent BS-MSC frames is performedaccording to this renewed value. This is called downlink FN slideprocessing.

Below explanation will be given of the detail of downlink FN slideprocessing referring to FIG. 35.

This processing proceeds as follows to recover sync, oncesynchronization of frames has been lost: when frames arriving with asufficiently long delay behind extraction timings at the downlink framereceiver 24-1 and downlink frame extraction controller 24-2 are detectedsuccessively, the radio frame number FNs given to those downlink framesby diversity handover trunk 34 are altered as appropriate to recoversync.

With this FN slide processing, it is necessary to prevent thediscrepancy of the radio frame number FNs of a plurality of BSs and theinformation dispatched to radio link. To prevent such discrepancy,adjustment of FN slide lengths between different BSs or slide timing maybe informed each other. In this example, however, the downlink FN slideprocessing is not performed by the downlink frame receiver 24-1 ofindividual BSs, but a BS which initially detects the appearance of delayinforms of it to the diversity handover trunk of information source, toallow the downlink frame FN adder 34-4 of diversity handover trunk toexecute the downlink slide processing. Then, detailed explanation willbe given below of both of BS and diversity handover trunk.

2.4.1. Processes in Base Station

In BS, user frames carrying predetermined radio frame number FN areextracted from a buffer according to the reference clock provided byBS-MSC frame synchronizer 21. When user frames which arrive behindextraction timings are detected by the downlink frame receiver 24-1 anddownlink frame extraction controller 24-2, downlink FN correctionrequest information is generated. The downlink FN correction requestinformation is sent by the uplink frame transmitter 24-10 by way of MIF23 through a user signal route to DTH of MSC. Alternatively, the sameinformation may be sent through a control signal route. For the lattercase, when user frames which arrive behind extraction timings aredetected, the downlink FN correction request is transmitted by way ofMDE of BS to PRC-B 22, and the same request is sent to PRC-M 32 ascontrol signals. Later, the downlink correction request is transferred,in MSC, from PRC-M32 to DHT controller 34-1 in DHT, and finally to thedownlink FN allocator where the downlink slide processing is executed toproduce a downlink FN correction request.

Advantages and disadvantages will be described below when the downlinkFN correction request is sent to a diversity handover trunk as controlsignals or as user signals. When it is sent as control signals,execution of it may increase the delay time or a load inflicted upon thecontrol processor. Alternatively, when it is sent as user signals, twopossible situations exist: the downlink FN slide request is applied touplink user frames received from some radio links, or it is sent asnotice dedicated user frames.

For the former situation, the FN slide request, if applied to a seriesof packets in which user frames are interrupted at intervals, may belost. While in the latter situation utilizing dedicated user frames fornotice, it ensures the transfer of request at a high speed and with anecessary timing, though increasing traffic. The notice dedicated userframes are called “downlink wired out-of-sync notice user frames”. Thewired out-of-sync notice user frame is transmitted independently ofuplink user frames. Further, the downlink wired out-of-sync notice userframe with downlink FN slide amount added thereto may be transmitted toa diversity handover trunk.

2.4.2. Function of Diversity Handover Trunk

For radio links, power control of signal transmission is performed onthe premise that all branches belonging to wired links contribute to thesynthetic gain of diversity handover. Accordingly, even when only onebranch among a plurality of branches dispatches a downlink FN sliderequest, the downlink frame FN adder 34-4 uses it as a trigger to startdownlink FN slide processing. When the downlink frame FN adder 34-4receives downlink wired out-of-sync notice user frames or a downlink FNslide request, it corrects the downlink frame number correction value bya certain amount (or by the downlink FN slide notified). The downlink FNslide width corrected in one processing is limited to be equal to orless than a predetermined FN slide decrement value, regardless of howmuch width is detected as a delay. Further, the total FN slide widthaccumulated between the onset and end of a stretch of communication islimited to be equal to or less than the predetermined maximum width fordownlink FN slide.

If the accumulated downlink FN slide width exceeds the maximum allowabledownlink FN slide width, DHT controller 34-1 informs of it to MSCprocessor 32. Being thus informed of an alarm notice, MSC processor 32gives a response, but MSC does not execute a downlink FN slideprocessing even if it has received meanwhile a downlink slide requestfrom BS, until MSC processor 32 gives the response. Namely, during thisinterval, the alarm warning the maximum allowable downlink FN slidewidth being overridden is ignored.

Parameters necessary for the downlink FN slide processing are listed inan FN slide processing parameter management table stored in MSCprocessor 32, and arranged in such an order as to allow choosing of aslide width and maximal allowable width appropriate for a given service,because choice of these parameters affects the quality of service duringcommunication. The downlink frame FN adder 34-4, after referring to theinformation in the table, executes the downlink FN slide processing. Forexample, when the service is concerned with voice communication, the FNslide width may be determined after due consideration has been given tothe delay nullifying capacity and lost frame compensating capacity ofVXC 35, while the maximal allowable slide width may be determined afterconsideration has been given to the effect of delay on voice.

When the service is concerned with data transmission, the effect due toframe loss can be minimized after due consideration has been given tothe frame cycle, as long as the delay nullifying capacity of DSC 36 isproperly considered and errors over a plurality of frames (for example,eight frames) are properly checked.

When FN slide width in one FN slide processing is determined to be equalto one FN slide width, and a delay in arrival at a receiving stationsexceeding that width arises, FN slide processing is executed a number oftimes. During the period while the succeeding FN slide proceedings areexecuted, communication is not interrupted owing to the out-of-syncstate of frames during the passage through the wired route. If diversityhandover is maintained, then communication is possible through anotherbranch in which no out-of-sync state exists in its wired route. Oneexample of FN slide processing parameter management table is shown inFIG. 32.

The outline of steps necessary for downlink FN slide processing will begiven with reference to FIG. 36. In FIG. 36 it is assumed that thesynchronization phase difference between frames through diversityhandover trunk 34 and those through BS2 is 0. BS 4 handles frames whichhave a synchronization phase difference from those handled by diversityhandover trunk 34, and thus the reference clock of BS 4 is by one clockunit (OFS) behind the corresponding reference clock of BS 2. It isfurther assumed that the maximal fluctuation delay frames may undergoduring the passage from diversity handover trunk 34 to BS is 38 msec(being equal to 23 line frame clocks (FN)+13 clock units (OFS)), beingthe same for BS 2 and BS 4

It is furthermore assumed that the downlink FN slide step width is “1”and the maximal downlink FN slide width is “5”. Then, as the maximalfluctuation delay is 38 msec, a frame to be extracted in BS 2 under thecondition of FN=6 and OFS =0 (at t2) corresponds with a frame dispatchedfrom diversity handover trunk 34 at FN=2 and OFS=3 (at t1).

However, in the example shown in the figure, the pertinent frame isdetected, in stead of time t2, at time t3 which is somewhat behind t2.On the other hand, in BS 4, a corresponding frame is detected at a righttiming (FN=5, OFS=15). In the former case, BS 2 sends a downlink wiredout-of-sync notice user frame to diversity handover trunk 34. This frameis received by diversity handover trunk 34 at a timing of FN=10 (at t4)(the wired out-of-sync notice user frame may be handled as soon as it isreceived, instead of its being handled as an ordinary frame according toits FN). Then, a slide processing is executed to determine what radioframe number FN is assigned to a frame coming immediately after t4.Namely, a frame transmitted at FN=10 and OFS=3 (at t5), which would beotherwise given FN=14, is given FN=15. Through these steps, a succeedingtrain of frames delivered from diversity handover trunk 34 to BS 2recovers synchronization.

Next, a detailed explanation will be given of uplink frame processing,referring to FIGS. 28 and 30.

2.5. Uplink Frame Processing in Base Station

In FIG. 3, when MS 1 uplink frames transmits, with BSs engaged indiversity handover, the radio transceiver 25 receives the uplink framesand sends them to the a frame receiver 24-5 in its MDE. In an uplinkframe extraction controller 24-6 of BS (BS 2 in above example) whichacts as a synchronization reference during the onset of communication,radio frames are extracted with the radio frame offset value ofreference clock being set to “0”. If there are no frames that can beextracted according to above timing, waiting time is prolonged to thenext timing (after “1” radio frame clock cycle) and extraction of framesis resumed.

In a subordinate BS, i.e., BS 4, radio frames are extracted at a timingafter a radio frame offset value OFS corresponding to thesynchronization phase difference (this is measured by an MS andbroadcast by MSC) from frames of BS 2 has been adjusted with respect tothe timing “0” of reference clock of BS 4. If the radio frame offsetvalue OFS thus obtained after fine adjustment extends over adjacentradio frame clocks, the radio frame number FN to be assigned to it isshifted in association (FIG. 28). Adjustment processing required bythese synchronization differences is the same with that seen in uplinkframes.

Return to FIG. 3. Radio frames thus extracted are provided to an uplinkframe processor 24-7, where encoding treatment for prevention of theentry of errors during transmission over radio zones and modulation forradio transmission are executed, to establish radio frames. In addition,the uplink frame processor 24-7 evaluates the received state of radioframes and scores it as a quality parameter. Then, an uplink framereliability information assignor 24-8 assigns the score or qualityparameter to BS-MSC frames.

These BS-MSC radio frames are provided to an uplink frame FN adder 24-9where the BS-MSC frames are given radio frame number FNs. The radioframe number FN given here is equal to the FN of reference clockprovided by the radio frame synchronizer 21 of BS.

In a subordinate BS, however, when the radio frame number FN is shiftedas a result of fine synchronization adjustment for a given series ofradio frames, shifted radio frame number FNs are given. BS-MSC frameswith radio frame number FNs attached are provided, by way of an uplinkframe transmitter 24-10, to MSC interface 23 of BS and further to MSC 3.

2.6. Uplink Frame Processing in MSC 3.

Next, in FIG. 2, an uplink frame receiver 34-7 of diversity handovertrunk 34 receives BS-MSC frames from BSs.

An uplink frame extraction controller 34-8 receives BS-MSC frames fromthe uplink frame receiver, extracts from them, based on DHO branchinformation (FIG. 4) provided by DHT controller 34-1, special frameswhich have the connection identifiers corresponding with activebranches, and have radio frame number FNs which are obtained after thereference clock provided by MSC radio frame synchronizer 31 has beencorrected according to the uplink frame number correction value, andsends them to an uplink frame comparator 34-9. When the received frameis a downlink wired out-of-sync notice user frame, it is delivered toDTH controller 34-1.

Extraction here is achieved according to the timing determined on thebasis of an uplink radio frame offset correction value provided by DHTcontroller 34-1. This timing adjustment is introduced to allowextraction to take place, making allowance for a fluctuation delaygenerated during transmission between BS and MSC, and frame shiftspossibly introduced in the processing performed by the uplink frame FNadder 24-9.

In above example, the timing of extraction performed by the uplink frameextraction controller 34-8 is equal to the timing if the uplink frameoffset correction value is 13. Further, the frame number FN assigned toBS-MSC frames to be extracted are equal to the frame number FN ofreference clock provided by MSC radio frame synchronizer 31 minus thedownlink frame number correction value three provided by DHT controller34-1 (FIG. 30).

MSC 3 monitors radio frame number FNs given to BS-MSC frames and storedin the buffer of uplink frame receiver 34-7. Whenever it detects acontinuously recurring delay in the arrival of BS-MSC frames carryingradio frame number FNs to be extracted, it concludes that a frame delayoccurs, dispatches a BS-MSC frame sync correction notice to DTHcontroller, and alters the uplink radio frame number correction value.

Through this process, radio frame number FNs assigned to subsequentframes are properly altered. This processing is called “uplink FN slideprocessing.” The extraction frequency (the number of extracted cells andcell interval when BS-MSC frames are transmitted in ATM mode) of BS-MSCframes is determined according to traffic information provided by DTHcontroller 34-1.

Then, a detailed explanation will be given to uplink FN slideprocessing.

This processing is, when frames are detected by the uplink framereceiver 34-7 and uplink frame extraction controller 34-8 that arrivebehind the extraction timing, to recover synchronization of those framesso that they may be transmitted from MSC to BS in an synchronizationstate.

For radio links, power control of signal transmission is performed onthe premise that all branches belonging to wired zones contribute to thesynthetic gain of diversity handover. Accordingly, even when only onebranch among a plurality of branches receives delayed frames, this delayis used as a trigger for uplink FN slide processing. If two or morebranches receive delayed frames, uplink FN slide processing is performedin accordance with frames with the largest delay.

Parameters used in uplink FN slide processing include an uplink FN slidewidth (uplink FN slide unit) which is given each time processing isperformed regardless of the amount of detected delay, and the maximal FNslide width (maximal allowable FN slide width) that uplink FN slideunits accumulated from the onset of communication to its end can take.

If accumulated uplink FN slide units exceed the maximal allowable uplinkFN slide width, DHT controller 34-1 gives an alarm warning the maximalallowable uplink FN slide width being overridden, to MSC processor 32.Being thus informed of an alarm notice, MSC processor 32 gives aresponse, but MSC does not execute an uplink FN slide processing even ifit has detected meanwhile a delay in frame transmission, until MSCprocessor 32 gives the response. Namely, during this interval, the alarmwarning the maximal allowable uplink FN slide width being overridden isignored.

Parameters necessary for the FN slide processing are listed in an FNslide processing parameter management table stored in MSC processor 32,being classified in terms of services. Thus, the uplink frame extractioncontroller 34-8 executes uplink FN slide processing after referring tothe information there. One example of a table listing parametersnecessary for FN slide processing is given in FIG. 32.

Steps required for uplink FN slide processing are outlined in FIGS. 33and 34. In FIG. 34, thin solid lines indicate the flow of frames with adelay within the maximal allowable limit during the transmission from BS4 to diversity handover trunk 34, while thick solid lines indicate theflow of frames whose delay exceeds the maximal allowable limit duringthe passage from BS 2 to diversity handover trunk 34.

The maximal fluctuation delay, synchronization errors of frames frominvolved BSs, and FN slide-parameters are set as follow. In BS 2described in downlink FN slide process, the frame having the framenumber FN=2 exceeds allowable limit. Therefore, if a normal control isexecuted, the frame of FN=3 will be extracted at the timing of FN=6 andOFS=13. However, in this case, the frame having the frame number FN=2will be extracted, because FN is shifted by “1”. If diversity handoveris maintained, and if overlapped extraction of a frame of FN=2 is to beavoided, extraction of one frame is skipped and renewed extraction maystarts from a frame of FN=3. Through this processing, it is possible forsubsequent frames from BS 2 to diversity handover trunk 34 to resumesynchronization.

Next, an uplink frame comparator 34-9 takes BS-MSC frames collected fromBSs engaged in diversity handover, refers to reliability data attachedto the radio frames, compares them, and executes the diversityselection. The detail of procedure will be explained with reference toFIG. 19.

FIG. 19 gives a radio frame number FN assigned to a BS-MSC frame incorrespondence with a radio frame, and, a list of reliability data. Thereliability data include radio frame out-of-sync evaluation bit (Sync),CRC evaluation bit (CRC), received SIR value (Con), level degradationevaluation bit (Level), and BER inferiority decision bit (BER). Reservebit (RES) is used to expand a given function. For example, this may beused to distinguish between a downlink wired out-of-sync notice userframe and an ordinary user frame.

Diversity selection achieved by the uplink frame comparator 34-9 takesplace according to the received SIR value and CRC evaluation bit. To bemore specific, of multiple BS-MSC frames whose CRC is OK, the one whosereceived SIR is the highest is chosen. When all candidate BS-MSC frameshave CRC judged to be NG, their bit data may be compared, ranked inorder of magnitude or submitted to calculation according to a certainevaluation function, and combined.

However, when the reliability data of wired frames from all involvedbranches contain a radio frame out-of-sync evaluation bit, a processingnecessary for meeting out-of-sync communication must be introduced. Thebasic steps necessary for selection process is shown in FIG. 21.

Then, an uplink frame analyzer 34-10 statistically calculates thetransmission quality of radio frames after selection of frame by frame,and when it finds that a given frame is so degraded as to meet astandard FER (Frame Error Rate), it dispatches a quality degradationalarm signal to MSC processor 32. Quality degradation evaluationparameters (FIG. 6) are given from diversity handover trunk 34 when acall is generated.

The uplink frame analyzer 34-10 also monitors radio frame out-of-syncevaluation bits, and each time it finds that radio frame out-of-syncoccurs N (N is a natural number) times in succession, it sends an alarmsignal warning out-of-sync communication to PRC-M. The number of timesat which out-of-sync wire frames occur in succession is provided by DHTcontroller. Here, referring to FIGS. 8-10, explanation will be given ofa simple method of quality evaluation based on the use of an up-downcounter.

Firstly, the basic working principle will be given with reference toFIG. 8. When N radio frames are received by one MSC from one or moreBSs, and those radio frames contain M degraded frames, FER of the framescan be expressed as M/N.

In FIG. 8, the FER quality measurement consists of checking, of N radioframes received, whether they contain two or more frames whose CRC isNG, and of, by so doing, ensuring that FEF of the radio frames is notmore than 1/N (FER≦1/N). To ensure FER≦1/6 for N=6, the counter is givenfive each time it receives a frame whose CRC is NG, while its number isdecreased by one each time it encounters with a frame whose CRC is OK.

A monitoring section checks that the number in counter does not exceedfive, thereby ensuring FER≦1/6. When N is alterable, and FER should bewithin 10⁻⁴, N=1000 is introduced into the counter and monitoring may beperformed in the same manner as above. If the quality standard is set toa high level, N will take a very large figure.

For example, when N=100,000 and a frame has a period of 10 ms, therequired monitoring time will be about 16 minutes (10 ms×100,000=about16 minutes). This will disrupt an effective monitoring even if themonitoring time is set far over an average holding time forcommunication. To meet this inconvenience, the counter is set to N=0 andis instructed to advance by one each time it receives a frame whose CRCis NG.

FIGS. 9 and 10 show flowcharts illustrating the steps of countingoperation incorporating above consideration. REPORT_(FER) is a thresholdat which, when the counter counts the number of degraded frames inexcess of a predetermined FER, and finds that the excess number reachesa certain value, it informs of the fact to PRC-M. This may be taken as ascale of protective steps which is required to reduce the announcementsto PRC-M when a given signal consists of frequently degraded frames.

REPORT_(SOUT) represents the number of out-of-sync frames occurring insuccession. This may be taken as a scale of protective steps becauseonly when the number of successive out-sync-frames after selectionexceeds this scale, a notice warning the occurrence of out-of-syncframes is dispatched.

Although FIGS. 8-10 give a quality measurement based on the use of anup-counter, other methods may be used for the measurement and detectionof quality and out-of-sync frames. For example, a window slide methodcan be mentioned where a window with a certain width is introduced andframes passing through the window are evaluated of their quality (inthis case parameters necessary for quality evaluation may be implementedin a different manner than above.)

Next, an uplink frame deliverer 34-11 attaches network side connectionidentifiers to intra-MSC frames, and the intra-MSC frames to a servicetrunk. The intra-MSC frames are transmitted to service trunks accordingto services appropriate for the frames (for examples, when the framescarry speech information, they are transmitted to a high efficiencyspeech coder 35, or when the frames carry data, they are transmitted toa data service control system 36).

The intra-MSC frames, after having been processed in an appropriateservice trunk, are transferred as relay frames to a relay network 12 byway of a relay network interface system 37, and routed to a target.However, when communication is made between different MSs, the servicetrunk may be bypassed as appropriate, in order to improve quality,delete delay, and minimize the consumption of trunk sources.

To add or remove branches engaged in diversity handover, MSC processor32 informs DHT controller 34-1 of the connection identifiers of branchesto be added or removed. Then, DHT controller 34-1 informs internalfunctional elements involved in the matter of the connection identifiersof branches to be added or removed. Through this action, processing inDHT is updated. The uplink frame analyzer 34-10 resets the previousresult of quality evaluation and restarts quality measurement.

Throughout the foregoing explanations regarding downlink frameprocessing, downlink FN slide processing, uplink frame processing anduplink FN slide processing, the timing of frame transmission orreception at the BS which acts as a sync reference is set to “0” or “15”for the simplicity of explanation, but, needless to say, the timing maybe set freely at will without interfering with the frame sync controldescribed above. The operator of a communication system, by setting thetiming to “0” or “15”, or at random, or deliberately according to acertain order, can distribute load evenly to involved systems, or routesevenly to involved stations, thereby achieving a statisticallysignificant multiple route efficiency.

2.7. Handover Control

Below explanation will be given of handover applied in mobilecommunication based on the use of diversity handover trunk 34.

Handover is classified from three aspects: (a) control range, (b)frequency, and (c) handover branches, and it will be explained fromthese aspects.

(a) Classification in Terms of Control Range

-   -   Handover classification in terms of control range is given in        FIG. 22.

Referring to FIG. 22, handover is roughly divided into two categories:handover practiced in one MSC, and handover practiced between differentMSCs (inter MSC handover).

The former handover or intra MSC handover is further divided intointra-cellular handover which is closed in one BS (or cell) andintercellular handover which covers different BSs (between differentcells). The intracellular handover is further divided, when BS ofinterest has a plurality of sectors, into intra-sector handover andinter-sector handover.

Handover between different MSCs or inter MSC handover is classified tointer sector handover. As seen from the network arrangement in FIG. 20,a peripheral MSC (MSC-V) is connected through an extended subscriberline to an anchor MSC (MSC-A), and diversity selection of frames isexecuted by MSC-A.

As shown in FIG. 38, when inter MSC handover is practiced, andcommunication between different MSCs is put into effect, delay intransmission is lengthened, and it becomes highly likely for the delayto exceed the fluctuation delay absorbing capacity of DHT. In this case,DHT executes above-described FN slide processing to recover sync offrames.

(b) Handover Classification in Terms of Frequency

-   -   Same frequency handover: handover of frames having the same        frequency    -   Different frequency handover: handover of frames having        different frequencies        (c) Handover Classification in Terms of Handover Branches        Involved    -   Diversity handover (DHO): handover with diversity state        maintained (addition, deletion and addition/deletion of        branches)    -   Branch switching handover: handover where all involved handover        branches are disconnected, and after a brief pause, a new set of        handover branches are entered for a renewed handover.    -   Re-connection type handover: frames from all involved handover        branches become out-of-sync, and after a brief interruption of        communication, a new set of branches are entered for a renewed        sync handover    -   Handover branch state classified by handover branch control is        given in FIG. 23.

One can identify a given handover by following, of the categories(a)-(c), which one it takes. (Example: intracellular, inter-sector,different frequency using, and branch switching handover, orintercellular, addition/deletion capable DHO handover, etc.)

The re-connection type handover is a mode by which, when communicationbetween MS and BS suffers out-of-sync, the network side reserves relaylines for a certain length of time, and the mobile station side searchesfor a BS which may reestablish lost sync. Thus, when the mobile stationfinds the announcement channel from a new BS (or a BS which itcommunicated previously) which may recover lost sync within that lengthof time, that mobile station is connected to the relay line reservedthus long.

Recall handover may be employed for the attainment of the same purpose.In this mode, the mobile station dispatches a recall includinginformation regarding previous communication state to a BS which canrecover promptly the previous communication state based on thisinformation.

FIGS. 24 and 25 are tables comparing handover triggers evoked in mobilecommunication, and handover types.

Three kinds of trigger assigned to big categories of narrowclassification in the left column of FIGS. 24 and 25 will be explainedbelow with respect to this example.

(1) DHO Trigger Due to Transmission Loss Measurement

Transmission loss is measured by MS for downlink frames. MS computestransmission loss by comparing the output power of its own sector and ofadjacent sectors which is provided through perch channels of sectorsengaged in communication, and the input power of signals received by MS.Then it arranges sectors in the ascending of transmission loss, convertsthe information into a cell condition report/handover trigger, and sendsit to MSC. (It adjusts announcement timing according to the timingdifference of sectors.)

As described earlier, DHO is a handover where site diversity ismaintained with base handover lines being kept closed and peripheralhandover lines with the same frequency bands being newly set while MSmoves over radio communication zones. It is possible to increase thecapacity of radio communication between adjacent sectors, bydistributing extra energy gained by improved quality of communicationdue to site diversity to transmission.

Addition/deletion of DHO branches may be determined according to thethreshold which is set for the difference between the transmission lossof branches engaged in communication, and the corresponding value ofbranches to be added/removed. (The threshold includes a threshold forDHO addition (DHO_ADD), DHO deletion (DHO_DEL), and branch switchinghandover (BHO_INI).)

Accordingly, diversity handover area is determined according to thetransmission loss between MS and BS as shown in FIG. 31.

If an MSC has an uplink frame interference level exceeding an allowablelimit, it can safely executes handover, because then the power necessaryfor transmitting uplink frames remains unchanged. However, if a downlinkframe interference level exceeds an allowable limit (maximaltransmission power permitted to BS), MSC can not execute handover.

In such case, MS does not execute handover, proceeds to an area wherehandover candidates reside, and causes degradation in communication ofother BSs existing in the same area. To avoid frequent occurrence ofsuch situation, it is necessary to limit the acceptance of calls to acertain level so that the capacity for handover calls may be keptsufficient. Later, MS passes through a diversity handover area, andmoves outside the zone where communication is in progress. Whencommunication quality is so degraded that it exceeds a threshold forBHO_INI, MS will execute BHO as will be described later.

(2) Branch Switching Handover Trigger

Branch switching handover is a handover where, when communicationdegradation intervenes, or MS passes a DHO area without resorting toDHO, and its communication quality is so degraded as to exceed athreshold for BHO_INI, base handover lines are opened while peripheralhandover lines are newly set. In the foregoing explanation of thetriggering of base handover lines with reference to FIGS. 24 and 25,both the outbreak of quality degradation and quality degradationsufficiently large to exceed a threshold for BHO_INI are said to benecessary for the execution of handover, but either one of the tworequirements may occur for the execution of handover.

Quality degradation measurement is executed by diversity handover trunk34 for uplink frames while it is done by MS for downlink frames. Belowquality degradation measurement performed by diversity handover trunk 34will be described.

Diversity handover trunk 34 statistically calculates the incidence ofNGs by checking CRC of user frames after diversity selection, and whenit finds that the measured FEF exceeds a threshold FER, it sends analarm signal telling quality degradation to MSC processor 32, whichstarts handover using the signal as a trigger.

To cite an example, branch switching handover is introduced when linesallocated for the same frequency band are in short of capacity, andlines allocated for a different frequency band has a sufficient capacityfor acceptance (acceptable in terms of capacity and availableresources), and otherwise squelch interruption of speech, or lineopening is executed. Limits of branch switching handover are determinedas shown in FIG. 31.

To cite another example, when MS in a diversity area finds no vacantcommunication channels (TRX) in BSs in its moving direction, MS does notexecute diversity handover. When it finds a blank communication channelnewly opened, it promptly starts diversity handover, but the frames ithandles exceed a limit of branch switching handover, it executes branchswitching handover.

When MS finds that BSs in its moving direction has no communicationchannels having the same frequency with that of the frames MS handles,it does not request diversity handover, but the frames it handles exceeda limit of branch switching handover, it executes branch switchinghandover.

Further, when MS remains in a certain zone and finds the capacity oftransmission lines of all BSs involved in that zone fully saturated(transmission power for downlink frames is maximal or the transmissionpower for uplink frames exceeds an allowable limit), it can executebranch switching handover even if the frames it handles does not exceeda limit of branch switching handover.

(3) Re-connection Type Handover Trigger or Disconnection Due toDetection of Out-of-Sync Communication

When a station continues to make a communication with quality beingdegraded, and degradation proceeds so much for a certain length of time(detection of out-of-sync state), disconnection of communication ensues.When the user of station insists continuing the communication,re-connection type handover is set in. Re-connection type handover is acontrol consisting of switching radio links, while holding the samecall.

Detection of out-of-sync communication is performed by diversityhandover trunk 34 for uplink frames while the same is done by MS 1 fordownlink frames. Below, how out-of-sync uplink frames are detected bydiversity handover trunk 34 will be described.

Each involved BS, whenever it detects out-of-sync radio frames in itsradio route, informs MSC 3 of the out-of-sync state as soon as theout-of-sync state surpasses protective steps. This information is givenin the form of radio frame out-of-sync evaluation bit contained in thereliability data of user frames.

Diversity handover trunk 34 monitors radio frame out-of-sync evaluationbits, and each time it finds that the occurrence of radio frameout-of-syncs exceeds REPORT_(SOUT), it sends an alarm signal warning theoccurrence of out-of-sync communication to MSC processor 32. MSCprocessor 32 starts re-connection type handover using the alarm as atrigger, or disconnects the call.

For appropriate handover to be set in various situations as describedabove, BS and MS have following functions.

BS constantly monitors the interference level of uplink frames and thetotal power consumed for transmission, and inserts, into broadcastinformations, their values together with their comparisons withcorresponding thresholds. BS sets thresholds separately for handover andreceipt/transmission of signals, because it respects handover more thanoriginating and terminating of calls. The thresholds for originating andterminating of calls are preferably set to a sterner levels than thatgiven to handover.

MS is provided with a function to monitor incoming broadcast informationduring waiting or communicating, and can determine by itself whether itis possible to currently execute originating and terminating of calls orhandover. MS receives a signal from an adjacent perch channel having thesame frequency band with that used for the communication in progress.Then, regarding a interference level to uplink, it computes transmissionloss on the basis of the transmission power through the perch channelwhich is derived from the broadcast information, and of receiving fieldlevel of the perch channel. Then, MS communicates with a BS which givesthe least transmission loss. Furthermore, MS compares transmissionlosses with interference levels to uplink frames in communications withadjacent BSs, and determines a zone to which it moves.

The sequence of steps necessary for diversity handover controlprocessing is shown in FIGS. 11 and 12, and the sequence of stepsnecessary for branch switching handover control processing is shown inFIGS. 13 and 14. Firstly, the sequence of steps necessary for diversityhandover control processing will be described. This is to ensureexecution of handover such that communication remains uninterrupted evenwhen MS moves from a zone governed by BS 2 (BS 1) to a zone governed byBS 4 (BS 2).

<Addition of Branches>

-   (1) When MS detects a branch (or branches) with a low transmission    loss, it measures the sync phase difference between radio frames    received by the reference branch or MS in communication, and radio    frames received by the branch to be added, and dispatches a request    for addition of a branch to MSC 3.-   (2) MSC 3 determines an appropriate one out of candidate branches,    asks BS 4 (BS 2) which governs the branch to be added whether the    branch has a sufficient resource such as radio routes and others,    and receives an affirmative answer. This step may be conglomerated    with the step (4).-   (3) MSC processor 32 informs diversity handover trunk 34 of a    request for addition of a branch, and sets diversity handover trunk    to be responsive to the request.-   (4) MSC 3 instructs BS 4 (BS 2) to set properly wired links between    MSC 3 and BS 4, and radio links.-   (5) BS 4 sets properly wired links, starts to transmission through    the downlink and to receives uplink frames, and returns a response    to MSC 3. At this stage, however, frames handled by BS 4 do not    always have a sync relation with frames handled by MS (this is    particularly true when the control of power for transmission uplink    frames by MS is directed to a BS other than BS 4).-   (6) MSC 3 instructs MS to add a new branch.-   (7) MS returns, to MSC 3, response to the instruction for addition    of a new branch.-   (8) MS adds the branch in question on a maximal-ratio combining    basis, and enters diversity handover. The steps (7) and (8) may be    exchanged in order.    <Deletion of Branch>-   (9) When MS detects a branch (or branches) which does not contribute    to the maximal-ratio combining, it sends a request for deletion of    the branch to MSC 3.-   (10) MSC 3 instructs MS to delete the branch.-   (11) MS execute deletion of the branch.-   (12) MSC 3 instructs BS 2 (BS 1) to delete previous radio and wired    routes.-   (13) BS 2 opens radio and wired routes, and informs of it to MSC.-   (14) MSC 3 informs of the order of branch deletion to diversity    handover trunk 34.

Next, explanation will be given of the sequence of steps necessary forbranch switching handover (FIGS. 13 and 14).

This is to ensure execution of handover with an interruption, when MSmoves from an area governed by BS 2 to another area governed by BS 4,and during the movement it does not resort to handover for some reasonand thus suffers degradation in communication, or degraded communicationexceeds a BHO threshold.

-   (1) When BS detects a branch with a low transmission loss, or a    branch (or branches) to which communication may be switched, it    measures the sync phase difference of loss of that branch from the    corresponding one of a referential branch, and informs of the result    as a report of cell condition to MSC 3 periodically or at intervals    whenever the state changes. MSC 3 memorizes the report.-   (2) When BS or diversity handover trunk 34 detects degraded    communication, handover destination branch is determined according    to the cell conditions of MS stored in the memory of MSC 3.-   (3) MSC 3 asks BS 4 which governs the branch to be switched to    whether the branch has a sufficient resource such as radio links and    others, and receives an affirmative answer. This step may be    conglomerated with the step (5).-   (4) MSC processor 32, informs diversity handover trunk 34 of a    request for addition of a branch, and sets diversity handover trunk    34 to be responsive to the request.-   (5) MSC 3 instructs BS 4 to set properly wired links between MSC 3    and BS 4, and radio links.-   (6) BS 4 sets properly wired links, starts to deliver uplink frames    through a radio link, and returns a response to MSC 3.-   (7) MSC 3 instructs MS to execute switching of branches.-   (8) MS disconnects communication with a previous branch and starts    to communicate with a new branch.-   (9) BS 4 checks that communication is established between MS and the    new branch, and informs MSC 3 that a synchronization state has been    established in the communication between MS and the new branch.-   (10) When MSC 3 receives the report from BS 4 that a sync state has    been established in the new communication, it instructs BS 2 to    release previous radio and wired links.-   (11) BS 2 releases the previous radio and wired roots in question,    and informs of it to MSC 3.-   (13) MSC 3 informs of the order of branch deletion to diversity    handover trunk 34.

In the sequence of steps depicted in FIGS. 11-14, commands for branchaddition and deletion is exchanged between MSC processor 32 anddiversity handover trunk 34. Information exchanged between the twoelements during the onset/end of communication and receipt/dispatch of areport informing degraded communication/outbreak of out-of-sync state isshown in FIGS. 15 and 16.

Information flow during onset of communication will be firstlydescribed.

MSC processor 32, when it receives a call, (1) recognizes the type ofservice, (2) determines the connection identifier, (3) computes timingcorrection parameters, (4) determines quality degradation measurementparameters, (5) determines out-of-sync state detection parameters, (6)analyzes traffic information, and informs the parameters obtained in thesteps (2)-(6) to DHT together with a DHT setting instruction command.

The diversity handover trunk 34 sets various inner condition accordingto the commands and parameters supplied thereto, and starts diversityhandover operation.

Next, information flow during the onset of handover will be described.

MSC 32, during addition or deletion of a wired branch, (7) determinesDHO connection identifier of the branch to be added or deleted, andinforms of the result to diversity handover trunk 34 together with acommand instructing addition or deletion of a branch.

Diversity handover trunk 34 updates the state in system according to thecommand and parameter it has received, and initiates a renewed diversityhandover with the new branch added.

To disconnect a given call, MSC processor 32 sends an instruction foropening the involved route to diversity handover trunk 34.

When degraded communication or out-of-sync state arises, diversityhandover trunk 34 dispatches an alarm signal to MSC processor 32 whichexecutes an appropriate treatment according to the content delivered bythe signal.

3. Advantages of Embodiment

Based on features as detailed above, this embodiment will bringfollowing advantages.

-   (1) In this embodiment, a common synchronization timing is ensured    in communication between MSs, BSs and MSCs. Frame identification    information is exchanged only between BS and MSC, and delays of    frame transmission different from one BS to another are nullified by    MSC and BS involved. Further, MS can receive radio frames from    different BSs at a synchronization timing, it manages communication    with a small capacity buffer. As frame identification information is    exchanged only between MSC and BS, and is not exchanged through    radio links, an efficient use of the radio transmission capacity is    ensured.-   (2) In this embodiment, during the onset of communication, a    communication controller informs of a rightly measured transmission    delay to a frame receiving system, and a frame extraction controller    extracts frames according to the type of service involved. Thus, it    is possible to achieve communication with a properly set    transmission delay according to the type of service.-   (3) In this embodiment, when the frame extractor detects an    out-of-sync state of received frames, it shifts the timing of    extracting frames as appropriate according to the period of frames,    and, by so doing, recovers a synchronization state for subsequent    frames. Thus, it is possible to continue communication without    disconnection.-   (4) In this embodiment, quality degradation is evaluated after    selection process, and hence it is possible to activate handover    using the quality degradation as a trigger. This contributes to    improvement of communication quality.-   (5) In this embodiment, each BS informs of an out-of-sync state to a    diversity handover trunk through a communication link, and allows    the diversity handover trunk to evaluate the out-of-sync state and    then to dispatch the result to an involved processor. Thus, it is    possible to reduce the amount of signals required when an    out-of-sync notice is directly dispatched to the processor as in a    conventional system, and thus load imposed on the processor.    4. Variations or Modifications

The present invention can be put into practice in various forms withoutencroaching the spirit or principal characteristics inherent thereto.Thus, the aforementioned embodiment is only illustrative in any respect,and should not be taken as restrictive of the present invention. Thescope of the present invention is only limited by what is defined byattached claims, and is never restricted by any description contained inthe text of Specification. Further, variations and modificationsequivalent to any claim are of course within the scope of the presentinvention.

For example, in above embodiment, clock errors and fluctuations intransmission delay of individual nodes are assumed to be known. Thepresent invention, however, can be applied to various cases: a casewhere the clocks of transmitter and receiver are not synchronized, acase where a fluctuation in transmission delay arising as a result ofsignals passing through a transmitter and receiver remains unknown, etc.

The operations according to above situations will be described below. InFIG. 37, a transceiver 100 has a clock circuit 101 to generate clockpulses CL1, and a receiver 120 has a clock circuit 102 to generate clockpulses CL2. The clock pulses CL1 and CL2 are not synchronized.Furthermore, the maximal delay due to fluctuations during the passage ofsignals between the transmitter and receiver 100 and 120 is assume to beunknown. The technique will be described, in which the receiver 120synchronizes the frames transmitted by transmitter 100.

First, transmitter 100 attaches the phase of clock pulses CL1 to framesas the radio frame number FN, before it transmits those frames. Receiver120 receives those frames, reads frame numbers FN attached to theframes, calculates the phase difference of a given frame number from acorresponding clock signal CL2. This calculation was repeated one ormore times for frames transmitted by a previous transmitter, the maximaldifference was obtained, and a safety factor was added thereto to give acorrection value which was then stored in a memory. From frames comingthereafter, the receiver extracts appropriate frames according to clockpulses CL2 and the correction value. This correction value can bechanged any time, if necessary, according to the current history ofcommunication.

Next, operation of the above modification will be explained.

Transmitter 100 is going to send, for example, a frame when the clocksignal CL1 has a phase FN of “55,” and attaches the radio frame numberFN of “55” to the frame. If the receiver 120 finds that thecorresponding CL2 is at “60” of the clock signal, then the difference is5 (60−55=5). In the same manner, if the phase FN of clock signal CL1 is“62” when a frame is transmitted, and clock signal CL2 is at “5” whenthe frame is received, the difference is 7 (64+5−62=7), because radioframe numbers FN change in a cyclic manner between “0” to “63”.

If the safety factor is assumed to be “2”, then the largest difference“7” of the two measurements is added with “2”, and the correction value“9” is obtained. In the subsequent process, the receiver 120 extractsframes according to the correction value. For a third example, when aframe of interest is received by receiver 120 at “6” of clock signalCL2, the difference is 61 (6−9+64=61). Thus, a frame having FN=61 isextracted. For a fourth example where a frame of interest is received byreceiver 120 at “7” of clock signal CL2, a frame having FN=62 isextracted. In this way it is possible to maintain a synchronizationstate of frames between transmitter 100 and receiver 120.

In above embodiment, various trunks are put together and distributed toa single MSC as shown in FIG. 39 (case 1). The present invention canalso be applied to case 2 in the same figure where MSCs are assigned toseveral blocks, and trunks are separately distributed to those blocks.In the example depicted in the figure, MSC is composed of MSC-1 andMSC-2. In this case, further, the number and location of MSCs-1 are notlimited by any specific requirements: they may be located close to BSs,and a plurality of MSCs-1 may be connected to a single MSC-2.

1. A diversity handover trunk for communicating with a mobile stationthrough a plurality of base stations, comprising: an offset timingcontrol configured to set an offset transmission timing of a frame ofdata to be transmitted from the diversity handover trunk, the offsettransmission timing being based on a correction value to an arrivaltiming of the frame of data to be transmitted from the diversityhandover trunk to the plurality of base stations; and a transmitterconfigured to transmit the frame of data to the base stations, given theoffset transmission timing, to achieve synchronization of transmissionsof the frame data from the plurality of base stations to the mobilestation.
 2. A diversity handover trunk according to claim 1, wherein thecorrection value is indicative of a length of time the fame of data isexpected to take to arrive at one of the plurality of base stations. 3.A diversity handover trunk according to claim 1, wherein the correctionvalue is indicative of how late the frame of data is expected arrive atone of the plurality of base stations.
 4. A diversity handover trunkaccording to claim 1, wherein the correction value is indicative of anexpected transmission delay time of the frame of data.
 5. A diversityhandover trunk according to claim 1, wherein the correction value isindicative of an expected maximum transmission delay time of the frameof data among the plurality of base stations.
 6. A diversity handovertrunk according to claim 1, wherein the offset transmission timing is aforward offset transmission timing.
 7. A method implemented at adiversity handover trunk for communicating with a mobile station througha plurality of base stations, comprising: setting an offset transmissiontiming of a frame of data to be transmitted from the diversity handovertrunk, the offset transmission timing being based on a correction valueto an arrival timing of the frame of data to be transmitted from thediversity handover trunk to the plurality of base stations; andtransmitting the frame of data to the plurality of base stations, giventhe offset transmission timing, to achieve synchronization oftransmissions of the frame data from the plurality of base stations tothe mobile station.
 8. A method according to claim 7, wherein thecorrection value is indicative of a length of time the fame of data isexpected to take to arrive at one of the plurality of base stations. 9.A method according to claim 7, wherein the correction value isindicative of how late the frame of data is expected arrive at one ofthe plurality of base stations.
 10. A method according to claim 7,wherein the correction value is indicative of an expected transmissiondelay time of the frame of data.
 11. A method according to claim 10,wherein the correction value is indicative of an expected maximumtransmission delay time of the frame of data among the plurality of basestations.
 12. A method according to claim 7, wherein the offsettransmission timing is a forward offset transmission timing.
 13. Adiversity handover trunk for communicating with a mobile station througha plurality of base stations, comprising: means for setting an offsettransmission timing of a frame of data to be transmitted from thediversity handover trunk, the offset transmission timing being based ona correction value to an arrival timing of the frame of data to betransmitted from the diversity handover trunk to the plurality of basestations; and means for transmitting the frame of data to the pluralityof base stations, given the offset transmission timing, to achievesynchronization of transmissions of the frame data from the plurality ofbase stations to the mobile station.
 14. A diversity handover trunkaccording to claim 13, wherein the correction value is indicative of alength of time the fame of data is expected to take to arrive at one ofthe plurality of base stations.
 15. A diversity handover trunk accordingto claim 13, wherein the correction value is indicative of how late theframe of data is expected arrive at one of the plurality of basestations.
 16. A diversity handover trunk according to claim 13, whereinthe correction value is indicative of an expected transmission delaytime of the frame of data.
 17. A diversity handover trunk according toclaim 13, wherein the correction value is indicative of an expectedmaximum transmission delay time of the frame of data among the pluralityof base stations.
 18. A diversity handover trunk according to claim 13,wherein the offset transmission timing is a forward offset transmissiontiming.